Electronic instruments produce electronic or electromagnetic interference (EMI), which can interfere with the operation of other electronic instruments. EMI is often modeled as two superposed current components: common-mode current and differential-mode current. Differential-mode currents, by definition, travel in opposite directions on a pair (or multiplicity) of conductors. Commonmode currents are defined as traveling in the same direction on a pair (or multiplicity) of conductors.
Differential-mode noise travels in the same direction as the desired signal, e.g., the drive signal or power component. In contrast, common-mode noise is expressed as a displacement with respect to a reference potential on both (or a multiplicity of) conductors at the instrument location. If the instrument in question measures its signals with respect to a ground potential, common-mode noise may corrupt its measurement of the desired signal, as described below.
Rejection of all common-mode currents, i.e., noise, is desired. To do so, conventional EMI filters employ a common-mode choke on the hot and neutral conductors (or "lines") of a power cord. For safety purposes, the ground line from the wall outlet is connected to the chassis of the electronic equipment. The design of conventional EMI filters implicitly assumes that the ground or "protective earth" conductor is a low impedance means to which radio frequency noise may be diverted. In that regard, the existing body of EMI regulations teaches that the "third conductor" ("ground") is "datum" or zero volts at all frequencies. This is not the case, especially for frequencies in the passband of machines such as diagnostic ultrasound equipment.
When measured at frequencies greater than 100 KHz, the power line is not at earth-ground potential at the point of use. In fact, the ground conductor, as well as the power conductors, exhibit high impedance with respect to EMI impressed by other sources, and also form an excellent antenna for radiated signals in the RF band. Two mechanisms are responsible for this phenomenon. First, the ground conductor in the distribution system exhibits inductive series impedance as a function of its length. This impedance may be in the hundreds of ohms for frequencies of 1 MHz and higher. Second, noise is capacitively coupled from the power conductors (hot and neutral) to the ground conductor along the length of the power run. In addition, other equipment connected to the same power distribution system may inject noise into the ground conductor by virtue of the design of their EMI filters.
Ground conductor noise can have significant effects on sensitive equipment, such as ultrasound machines. The chassis of the ultrasound machine typically is connected to the ground conductor of the wall outlet. An extremely sensitive (1-10 nanovolt) ultrasound transducer array operating in the 0-20 MHz range is connected to the chassis through a shielded cable. The cable shielding is connected to the chassis ground, whereas the center conductors are connected to the transducer array elements. The noise voltage on the chassis ground is experienced at the transducer cable shielding and coupled by mutual inductance to the transducer element conductors. This results in noise on the desired ultrasound signal.
Such power line noise at the point of use has been found on the ground conductor in substantially the same amount as on the power conductors. However, conventional EMI filters in Class I equipment (grounded metallic chassis) connect the ground connector directly to the chassis or other grounded metal. This practice, as described above, impresses the noise from the ground conductor directly onto the chassis and negates any beneficial effect derived from the common-mode choke, which is typically in series only with the power conductors.
In an attempt to address this problem, some EMI filters incorporate a discrete inductor in series with the ground conductor between the equipment chassis and the equipment's connection to the ground line running from the wall outlet. Although this inductor mitigates the above problem, it actually can detract from the performance of the EMI filter in other modes because it inserts a high impedance in series with the ground conductor. First, noise may be present on the power conductors but not the ground conductor. In that case, the additional inductor reduces the effectiveness of the Y-capacitors (typically inserted between ground and each of the hot and neutral power conductors) by inserting an impedance in series with the ground conductor, thereby increasing the susceptibility of the equipment to noise. Second, the additional inductor can increase noise emissions on the power line wires, again because it reduces the effectiveness of the Y-capacitors by isolating them from the ground conductor.
To improve common-mode noise rejection on the ground line, some manufacturers, such as Almor Corporation, include a common-mode choke in the power cord. These chokes include a ferrite bead onto which one to four turns of a multiconductor power cord are wound.
Such in-line EMI filters are bulky and limited in performance. The large, heavy in-line core that is wound with a multiconductor cable is typically placed near the power plug, resulting in a tendency to pull the plug out of the wall socket due to its weight. If greater noise rejection were desired, then a higher impedance, and, thus, a greater number of turns would be necessary. This causes even more weight on the power cord. Moreover, if capacitors were used in conjunction with the in-line choke to improve performance, this addition would also increase size and weight. Further, although the in-line EMI filter achieves some common-mode noise rejection, the proximity of the hot, neutral and ground wires to each other causes mutual capacitance among those wires that prevents the filter from realizing its optimum common-mode noise rejection. Because the wires are wound in a trifilar fashion, the coupling coefficient K is nearly equal to 1, resulting in essentially no differential noise rejection.
Accordingly, it is desired to provide an improved EMI filter that achieves common-mode noise rejection without these limitations.